The group's leader, Rabbi Arik Ascherman, said that according to "the evidence we have received, it is pretty clear it was arson," but he would not say who might have carried out the attack.
At about 3 a.m., Israeli police received a report of a burning house in the village of Duma. The home belonged to Ibrahim Dawabsheh, who was taken to the hospital after suffering from smoke inhalation, according to Israeli police spokeswoman Luba Samri.
No one else was injured in the fire, Samri says.
Dawabsheh is the key witness to the arson attack in the same village that killed three members of his extended family in late July: 18-month-old Ali Dawabsheh and his parents, Saad and Reham Dawabsheh.
Their son, Ahmad, 5, suffered severe burns over 80% of his body, and has been undergoing months of hospital treatments.
A joint statement released Sunday evening from the Israel Security Agency and Israel police said, "There is a growing assessment that this was not nationalistic in nature. The initial findings at the scene are not consistent with the characteristics of deliberate arson by Jews."
All other details of the investigation remain under a gag order, the statement read.
Amiram Ben-Uliel, 21, a Jewish extremist known to Israeli authorities, was charged in the July firebombing. A minor was charged with being an accomplice to the attack.
In a statement issued Sunday morning, PLO Executive Committee member Dr. Hanan Ashrawi condemned the latest suspected arson, blaming the Israeli government.
"Since Israel and its army have done very little to prevent further settler violence against Palestinians or to hold the settlers accountable, we call on the international community to act urgently and to assume its responsibilities to protect the Palestinian people and to put an end to the impunity that Israel and its settlers continue to enjoy at the expense of Palestinian lives."